Commit 36697529 authored by Peter Hurley's avatar Peter Hurley Committed by Greg Kroah-Hartman

tty: Replace ldisc locking with ldisc_sem

Line discipline locking was performed with a combination of
a mutex, a status bit, a count, and a waitqueue -- basically,
a rw semaphore.

Replace the existing combination with an ld_semaphore.

Fixes:
 1) the 'reference acquire after ldisc locked' bug
 2) the over-complicated halt mechanism
 3) lock order wrt. tty_lock()
 4) dropping locks while changing ldisc
 5) previously unidentified deadlock while locking ldisc from
    both linked ttys concurrently
 6) previously unidentified recursive deadlocks

Adds much-needed lockdep diagnostics.
Signed-off-by: default avatarPeter Hurley <peter@hurleysoftware.com>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent d2c43890
......@@ -429,7 +429,7 @@ static void flush_to_ldisc(struct work_struct *work)
return;
disc = tty_ldisc_ref(tty);
if (disc == NULL) /* !TTY_LDISC */
if (disc == NULL)
return;
spin_lock_irqsave(&buf->lock, flags);
......
......@@ -1388,8 +1388,7 @@ static int tty_reopen(struct tty_struct *tty)
struct tty_driver *driver = tty->driver;
if (test_bit(TTY_CLOSING, &tty->flags) ||
test_bit(TTY_HUPPING, &tty->flags) ||
test_bit(TTY_LDISC_CHANGING, &tty->flags))
test_bit(TTY_HUPPING, &tty->flags))
return -EIO;
if (driver->type == TTY_DRIVER_TYPE_PTY &&
......@@ -1405,7 +1404,7 @@ static int tty_reopen(struct tty_struct *tty)
}
tty->count++;
WARN_ON(!test_bit(TTY_LDISC, &tty->flags));
WARN_ON(!tty->ldisc);
return 0;
}
......@@ -3017,7 +3016,7 @@ void initialize_tty_struct(struct tty_struct *tty,
tty->pgrp = NULL;
mutex_init(&tty->legacy_mutex);
mutex_init(&tty->termios_mutex);
mutex_init(&tty->ldisc_mutex);
init_ldsem(&tty->ldisc_sem);
init_waitqueue_head(&tty->write_wait);
init_waitqueue_head(&tty->read_wait);
INIT_WORK(&tty->hangup_work, do_tty_hangup);
......
......@@ -45,7 +45,6 @@ enum {
*/
static DEFINE_RAW_SPINLOCK(tty_ldiscs_lock);
static DECLARE_WAIT_QUEUE_HEAD(tty_ldisc_wait);
/* Line disc dispatch table */
static struct tty_ldisc_ops *tty_ldiscs[NR_LDISCS];
......@@ -153,7 +152,7 @@ static void put_ldops(struct tty_ldisc_ops *ldops)
* takes tty_ldiscs_lock to guard against ldisc races
*/
static struct tty_ldisc *tty_ldisc_get(int disc)
static struct tty_ldisc *tty_ldisc_get(struct tty_struct *tty, int disc)
{
struct tty_ldisc *ld;
struct tty_ldisc_ops *ldops;
......@@ -180,8 +179,7 @@ static struct tty_ldisc *tty_ldisc_get(int disc)
}
ld->ops = ldops;
atomic_set(&ld->users, 1);
init_waitqueue_head(&ld->wq_idle);
ld->tty = tty;
return ld;
}
......@@ -193,20 +191,11 @@ static struct tty_ldisc *tty_ldisc_get(int disc)
*/
static inline void tty_ldisc_put(struct tty_ldisc *ld)
{
unsigned long flags;
if (WARN_ON_ONCE(!ld))
return;
raw_spin_lock_irqsave(&tty_ldiscs_lock, flags);
/* unreleased reader reference(s) will cause this WARN */
WARN_ON(!atomic_dec_and_test(&ld->users));
ld->ops->refcount--;
module_put(ld->ops->owner);
put_ldops(ld->ops);
kfree(ld);
raw_spin_unlock_irqrestore(&tty_ldiscs_lock, flags);
}
static void *tty_ldiscs_seq_start(struct seq_file *m, loff_t *pos)
......@@ -257,34 +246,6 @@ const struct file_operations tty_ldiscs_proc_fops = {
.release = seq_release,
};
/**
* tty_ldisc_try - internal helper
* @tty: the tty
*
* Make a single attempt to grab and bump the refcount on
* the tty ldisc. Return 0 on failure or 1 on success. This is
* used to implement both the waiting and non waiting versions
* of tty_ldisc_ref
*
* Locking: takes tty_ldiscs_lock
*/
static struct tty_ldisc *tty_ldisc_try(struct tty_struct *tty)
{
unsigned long flags;
struct tty_ldisc *ld;
/* FIXME: this allows reference acquire after TTY_LDISC is cleared */
raw_spin_lock_irqsave(&tty_ldiscs_lock, flags);
ld = NULL;
if (test_bit(TTY_LDISC, &tty->flags) && tty->ldisc) {
ld = tty->ldisc;
atomic_inc(&ld->users);
}
raw_spin_unlock_irqrestore(&tty_ldiscs_lock, flags);
return ld;
}
/**
* tty_ldisc_ref_wait - wait for the tty ldisc
* @tty: tty device
......@@ -298,16 +259,15 @@ static struct tty_ldisc *tty_ldisc_try(struct tty_struct *tty)
* against a discipline change, such as an existing ldisc reference
* (which we check for)
*
* Locking: call functions take tty_ldiscs_lock
* Note: only callable from a file_operations routine (which
* guarantees tty->ldisc != NULL when the lock is acquired).
*/
struct tty_ldisc *tty_ldisc_ref_wait(struct tty_struct *tty)
{
struct tty_ldisc *ld;
/* wait_event is a macro */
wait_event(tty_ldisc_wait, (ld = tty_ldisc_try(tty)) != NULL);
return ld;
ldsem_down_read(&tty->ldisc_sem, MAX_SCHEDULE_TIMEOUT);
WARN_ON(!tty->ldisc);
return tty->ldisc;
}
EXPORT_SYMBOL_GPL(tty_ldisc_ref_wait);
......@@ -318,13 +278,18 @@ EXPORT_SYMBOL_GPL(tty_ldisc_ref_wait);
* Dereference the line discipline for the terminal and take a
* reference to it. If the line discipline is in flux then
* return NULL. Can be called from IRQ and timer functions.
*
* Locking: called functions take tty_ldiscs_lock
*/
struct tty_ldisc *tty_ldisc_ref(struct tty_struct *tty)
{
return tty_ldisc_try(tty);
struct tty_ldisc *ld = NULL;
if (ldsem_down_read_trylock(&tty->ldisc_sem)) {
ld = tty->ldisc;
if (!ld)
ldsem_up_read(&tty->ldisc_sem);
}
return ld;
}
EXPORT_SYMBOL_GPL(tty_ldisc_ref);
......@@ -334,27 +299,11 @@ EXPORT_SYMBOL_GPL(tty_ldisc_ref);
*
* Undoes the effect of tty_ldisc_ref or tty_ldisc_ref_wait. May
* be called in IRQ context.
*
* Locking: takes tty_ldiscs_lock
*/
void tty_ldisc_deref(struct tty_ldisc *ld)
{
unsigned long flags;
if (WARN_ON_ONCE(!ld))
return;
raw_spin_lock_irqsave(&tty_ldiscs_lock, flags);
/*
* WARNs if one-too-many reader references were released
* - the last reference must be released with tty_ldisc_put
*/
WARN_ON(atomic_dec_and_test(&ld->users));
raw_spin_unlock_irqrestore(&tty_ldiscs_lock, flags);
if (waitqueue_active(&ld->wq_idle))
wake_up(&ld->wq_idle);
ldsem_up_read(&ld->tty->ldisc_sem);
}
EXPORT_SYMBOL_GPL(tty_ldisc_deref);
......@@ -437,27 +386,6 @@ static void __lockfunc tty_ldisc_enable_pair(struct tty_struct *tty,
tty_ldisc_unlock_pair(tty, tty2);
}
/**
* tty_ldisc_enable - allow ldisc use
* @tty: terminal to activate ldisc on
*
* Set the TTY_LDISC flag when the line discipline can be called
* again. Do necessary wakeups for existing sleepers. Clear the LDISC
* changing flag to indicate any ldisc change is now over.
*
* Note: nobody should set the TTY_LDISC bit except via this function.
* Clearing directly is allowed.
*/
static void tty_ldisc_enable(struct tty_struct *tty)
{
clear_bit(TTY_LDISC_HALTED, &tty->flags);
set_bit(TTY_LDISC, &tty->flags);
clear_bit(TTY_LDISC_CHANGING, &tty->flags);
wake_up(&tty_ldisc_wait);
}
/**
* tty_ldisc_flush - flush line discipline queue
* @tty: tty
......@@ -555,14 +483,14 @@ static void tty_ldisc_restore(struct tty_struct *tty, struct tty_ldisc *old)
int r;
/* There is an outstanding reference here so this is safe */
old = tty_ldisc_get(old->ops->num);
old = tty_ldisc_get(tty, old->ops->num);
WARN_ON(IS_ERR(old));
tty->ldisc = old;
tty_set_termios_ldisc(tty, old->ops->num);
if (tty_ldisc_open(tty, old) < 0) {
tty_ldisc_put(old);
/* This driver is always present */
new_ldisc = tty_ldisc_get(N_TTY);
new_ldisc = tty_ldisc_get(tty, N_TTY);
if (IS_ERR(new_ldisc))
panic("n_tty: get");
tty->ldisc = new_ldisc;
......@@ -575,101 +503,6 @@ static void tty_ldisc_restore(struct tty_struct *tty, struct tty_ldisc *old)
}
}
/**
* tty_ldisc_wait_idle - wait for the ldisc to become idle
* @tty: tty to wait for
* @timeout: for how long to wait at most
*
* Wait for the line discipline to become idle. The discipline must
* have been halted for this to guarantee it remains idle.
*/
static int tty_ldisc_wait_idle(struct tty_struct *tty, long timeout)
{
long ret;
ret = wait_event_timeout(tty->ldisc->wq_idle,
atomic_read(&tty->ldisc->users) == 1, timeout);
return ret > 0 ? 0 : -EBUSY;
}
/**
* tty_ldisc_halt - shut down the line discipline
* @tty: tty device
* @o_tty: paired pty device (can be NULL)
* @timeout: # of jiffies to wait for ldisc refs to be released
*
* Shut down the line discipline and work queue for this tty device and
* its paired pty (if exists). Clearing the TTY_LDISC flag ensures
* no further references can be obtained, while waiting for existing
* references to be released ensures no more data is fed to the ldisc.
*
* You need to do a 'flush_scheduled_work()' (outside the ldisc_mutex)
* in order to make sure any currently executing ldisc work is also
* flushed.
*/
static int tty_ldisc_halt(struct tty_struct *tty, struct tty_struct *o_tty,
long timeout)
{
int retval;
clear_bit(TTY_LDISC, &tty->flags);
if (o_tty)
clear_bit(TTY_LDISC, &o_tty->flags);
retval = tty_ldisc_wait_idle(tty, timeout);
if (!retval && o_tty)
retval = tty_ldisc_wait_idle(o_tty, timeout);
if (retval)
return retval;
set_bit(TTY_LDISC_HALTED, &tty->flags);
if (o_tty)
set_bit(TTY_LDISC_HALTED, &o_tty->flags);
return 0;
}
/**
* tty_ldisc_hangup_halt - halt the line discipline for hangup
* @tty: tty being hung up
*
* Shut down the line discipline and work queue for the tty device
* being hungup. Clear the TTY_LDISC flag to ensure no further
* references can be obtained and wait for remaining references to be
* released to ensure no more data is fed to this ldisc.
* Caller must hold legacy and ->ldisc_mutex.
*
* NB: tty_set_ldisc() is prevented from changing the ldisc concurrently
* with this function by checking the TTY_HUPPING flag.
*/
static bool tty_ldisc_hangup_halt(struct tty_struct *tty)
{
char cur_n[TASK_COMM_LEN], tty_n[64];
long timeout = 3 * HZ;
clear_bit(TTY_LDISC, &tty->flags);
if (tty->ldisc) { /* Not yet closed */
tty_unlock(tty);
while (tty_ldisc_wait_idle(tty, timeout) == -EBUSY) {
timeout = MAX_SCHEDULE_TIMEOUT;
printk_ratelimited(KERN_WARNING
"%s: waiting (%s) for %s took too long, but we keep waiting...\n",
__func__, get_task_comm(cur_n, current),
tty_name(tty, tty_n));
}
set_bit(TTY_LDISC_HALTED, &tty->flags);
/* must reacquire both locks and preserve lock order */
mutex_unlock(&tty->ldisc_mutex);
tty_lock(tty);
mutex_lock(&tty->ldisc_mutex);
}
return !!tty->ldisc;
}
/**
* tty_set_ldisc - set line discipline
* @tty: the terminal to set
......@@ -679,103 +512,47 @@ static bool tty_ldisc_hangup_halt(struct tty_struct *tty)
* context. The ldisc change logic has to protect itself against any
* overlapping ldisc change (including on the other end of pty pairs),
* the close of one side of a tty/pty pair, and eventually hangup.
*
* Locking: takes tty_ldiscs_lock, termios_mutex
*/
int tty_set_ldisc(struct tty_struct *tty, int ldisc)
{
int retval;
struct tty_ldisc *o_ldisc, *new_ldisc;
struct tty_struct *o_tty;
struct tty_struct *o_tty = tty->link;
new_ldisc = tty_ldisc_get(ldisc);
new_ldisc = tty_ldisc_get(tty, ldisc);
if (IS_ERR(new_ldisc))
return PTR_ERR(new_ldisc);
tty_lock(tty);
/*
* We need to look at the tty locking here for pty/tty pairs
* when both sides try to change in parallel.
*/
o_tty = tty->link; /* o_tty is the pty side or NULL */
retval = tty_ldisc_lock_pair_timeout(tty, o_tty, 5 * HZ);
if (retval) {
tty_ldisc_put(new_ldisc);
return retval;
}
/*
* Check the no-op case
*/
if (tty->ldisc->ops->num == ldisc) {
tty_unlock(tty);
tty_ldisc_enable_pair(tty, o_tty);
tty_ldisc_put(new_ldisc);
return 0;
}
mutex_lock(&tty->ldisc_mutex);
/*
* We could be midstream of another ldisc change which has
* dropped the lock during processing. If so we need to wait.
*/
while (test_bit(TTY_LDISC_CHANGING, &tty->flags)) {
mutex_unlock(&tty->ldisc_mutex);
tty_unlock(tty);
wait_event(tty_ldisc_wait,
test_bit(TTY_LDISC_CHANGING, &tty->flags) == 0);
tty_lock(tty);
mutex_lock(&tty->ldisc_mutex);
}
set_bit(TTY_LDISC_CHANGING, &tty->flags);
/*
* No more input please, we are switching. The new ldisc
* will update this value in the ldisc open function
*/
/* FIXME: why 'shutoff' input if the ldisc is locked? */
tty->receive_room = 0;
o_ldisc = tty->ldisc;
tty_unlock(tty);
/*
* Make sure we don't change while someone holds a
* reference to the line discipline. The TTY_LDISC bit
* prevents anyone taking a reference once it is clear.
* We need the lock to avoid racing reference takers.
*
* We must clear the TTY_LDISC bit here to avoid a livelock
* with a userspace app continually trying to use the tty in
* parallel to the change and re-referencing the tty.
*/
retval = tty_ldisc_halt(tty, o_tty, 5 * HZ);
/*
* Wait for hangup to complete, if pending.
* We must drop the mutex here in case a hangup is also in process.
*/
mutex_unlock(&tty->ldisc_mutex);
flush_work(&tty->hangup_work);
tty_lock(tty);
mutex_lock(&tty->ldisc_mutex);
/* handle wait idle failure locked */
if (retval) {
tty_ldisc_put(new_ldisc);
goto enable;
}
/* FIXME: for testing only */
WARN_ON(test_bit(TTY_HUPPED, &tty->flags));
if (test_bit(TTY_HUPPING, &tty->flags)) {
/* We were raced by the hangup method. It will have stomped
the ldisc data and closed the ldisc down */
clear_bit(TTY_LDISC_CHANGING, &tty->flags);
mutex_unlock(&tty->ldisc_mutex);
tty_ldisc_enable_pair(tty, o_tty);
tty_ldisc_put(new_ldisc);
tty_unlock(tty);
return -EIO;
......@@ -804,14 +581,10 @@ int tty_set_ldisc(struct tty_struct *tty, int ldisc)
tty_ldisc_put(o_ldisc);
enable:
/*
* Allow ldisc referencing to occur again
*/
tty_ldisc_enable(tty);
if (o_tty)
tty_ldisc_enable(o_tty);
tty_ldisc_enable_pair(tty, o_tty);
/* Restart the work queue in case no characters kick it off. Safe if
already running */
......@@ -819,7 +592,6 @@ enable:
if (o_tty)
schedule_work(&o_tty->port->buf.work);
mutex_unlock(&tty->ldisc_mutex);
tty_unlock(tty);
return retval;
}
......@@ -852,7 +624,7 @@ static void tty_reset_termios(struct tty_struct *tty)
static int tty_ldisc_reinit(struct tty_struct *tty, int ldisc)
{
struct tty_ldisc *ld = tty_ldisc_get(ldisc);
struct tty_ldisc *ld = tty_ldisc_get(tty, ldisc);
if (IS_ERR(ld))
return -1;
......@@ -891,14 +663,8 @@ void tty_ldisc_hangup(struct tty_struct *tty)
tty_ldisc_debug(tty, "closing ldisc: %p\n", tty->ldisc);
/*
* FIXME! What are the locking issues here? This may me overdoing
* things... This question is especially important now that we've
* removed the irqlock.
*/
ld = tty_ldisc_ref(tty);
if (ld != NULL) {
/* We may have no line discipline at this point */
if (ld->ops->flush_buffer)
ld->ops->flush_buffer(tty);
tty_driver_flush_buffer(tty);
......@@ -909,21 +675,22 @@ void tty_ldisc_hangup(struct tty_struct *tty)
ld->ops->hangup(tty);
tty_ldisc_deref(ld);
}
/*
* FIXME: Once we trust the LDISC code better we can wait here for
* ldisc completion and fix the driver call race
*/
wake_up_interruptible_poll(&tty->write_wait, POLLOUT);
wake_up_interruptible_poll(&tty->read_wait, POLLIN);
tty_unlock(tty);
/*
* Shutdown the current line discipline, and reset it to
* N_TTY if need be.
*
* Avoid racing set_ldisc or tty_ldisc_release
*/
mutex_lock(&tty->ldisc_mutex);
tty_ldisc_lock_pair(tty, tty->link);
tty_lock(tty);
if (tty_ldisc_hangup_halt(tty)) {
if (tty->ldisc) {
/* At this point we have a halted ldisc; we want to close it and
reopen a new ldisc. We could defer the reopen to the next
......@@ -942,9 +709,8 @@ void tty_ldisc_hangup(struct tty_struct *tty)
BUG_ON(tty_ldisc_reinit(tty, N_TTY));
WARN_ON(tty_ldisc_open(tty, tty->ldisc));
}
tty_ldisc_enable(tty);
}
mutex_unlock(&tty->ldisc_mutex);
tty_ldisc_enable_pair(tty, tty->link);
if (reset)
tty_reset_termios(tty);
......@@ -976,15 +742,12 @@ int tty_ldisc_setup(struct tty_struct *tty, struct tty_struct *o_tty)
tty_ldisc_close(tty, ld);
return retval;
}
tty_ldisc_enable(o_tty);
}
tty_ldisc_enable(tty);
return 0;
}
static void tty_ldisc_kill(struct tty_struct *tty)
{
mutex_lock(&tty->ldisc_mutex);
/*
* Now kill off the ldisc
*/
......@@ -995,7 +758,6 @@ static void tty_ldisc_kill(struct tty_struct *tty)
/* Ensure the next open requests the N_TTY ldisc */
tty_set_termios_ldisc(tty, N_TTY);
mutex_unlock(&tty->ldisc_mutex);
}
/**
......@@ -1017,15 +779,16 @@ void tty_ldisc_release(struct tty_struct *tty, struct tty_struct *o_tty)
tty_ldisc_debug(tty, "closing ldisc: %p\n", tty->ldisc);
tty_ldisc_halt(tty, o_tty, MAX_SCHEDULE_TIMEOUT);
tty_ldisc_lock_pair(tty, o_tty);
tty_lock_pair(tty, o_tty);
/* This will need doing differently if we need to lock */
tty_ldisc_kill(tty);
if (o_tty)
tty_ldisc_kill(o_tty);
tty_unlock_pair(tty, o_tty);
tty_ldisc_unlock_pair(tty, o_tty);
/* And the memory resources remaining (buffers, termios) will be
disposed of when the kref hits zero */
......@@ -1042,7 +805,7 @@ void tty_ldisc_release(struct tty_struct *tty, struct tty_struct *o_tty)
void tty_ldisc_init(struct tty_struct *tty)
{
struct tty_ldisc *ld = tty_ldisc_get(N_TTY);
struct tty_ldisc *ld = tty_ldisc_get(tty, N_TTY);
if (IS_ERR(ld))
panic("n_tty: init_tty");
tty->ldisc = ld;
......
......@@ -238,7 +238,7 @@ struct tty_struct {
int index;
/* Protects ldisc changes: Lock tty not pty */
struct mutex ldisc_mutex;
struct ld_semaphore ldisc_sem;
struct tty_ldisc *ldisc;
struct mutex atomic_write_lock;
......@@ -305,8 +305,6 @@ struct tty_file_private {
#define TTY_DO_WRITE_WAKEUP 5 /* Call write_wakeup after queuing new */
#define TTY_PUSH 6 /* n_tty private */
#define TTY_CLOSING 7 /* ->close() in progress */
#define TTY_LDISC 9 /* Line discipline attached */
#define TTY_LDISC_CHANGING 10 /* Line discipline changing */
#define TTY_LDISC_OPEN 11 /* Line discipline is open */
#define TTY_PTY_LOCK 16 /* pty private */
#define TTY_NO_WRITE_SPLIT 17 /* Preserve write boundaries to driver */
......
......@@ -203,8 +203,7 @@ struct tty_ldisc_ops {
struct tty_ldisc {
struct tty_ldisc_ops *ops;
atomic_t users;
wait_queue_head_t wq_idle;
struct tty_struct *tty;
};
#define TTY_LDISC_MAGIC 0x5403
......
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